Electron beam heating system

ABSTRACT

An electron beam heating system for an electron beam furnace is described in which material contained within a skulled crucible is heated. At least four and less than nine elongated fialments of thermionically emissive material are supported parallel with each other within an elongated open-sided channel defined by a focusing electrode. The filaments extend along the length of the channel and are spaced from the focusing electrode. The filaments are heated and electrons emitted thereby are accelerated by an accelerating electrode.

United States Patent 1 3,409,729 ll/1968 Hankset al .L Q. 13/31 Claims, 3 Drawing Figures 7 Tsujimoto [4 1 July 24,1973

[54] ELECTRON BEAM HEATING SYSTEM 3,701,915 10/1972 Tsujimoto 313/82 [75] Inventor: .Kaiunil N. Tsujimoto, El Cerrito, Primary Examiner-Roy N. Envall, Jr.

Calif. Attorney -William B. Anderson, R. Steven linkstaff l [73] Assignee: Alrco, inc New York, N.Y. and Bruce Mccubbrey cull 221 Filed: May 26, 1972 [571 ABSTRACT r An electron beam heating system for an electron beam [21] Appl' 257429 fumace is described in which material contained within a skulled crucible is heated. At least four and less than [52] us. Cl. 13/31, 313/82 nine gat fia m nts 0f thermionically emissive [51] Int. Cl. 1105b 7/00 material are supp d par with each ther within [58] Field of Search 13/31; 313/82 an ng t open-sided channel defined by a focusing electrode. The filaments extend along the length of the [S6] 7 Referen Cit d channel and are-spacedfrom the focusing electrode. UNITED STATES PATENTS The filaments are heated and electrons emitted thereby 172 007 3,1965 Hanks et 3] 13/31 X are accelerated by an accelerating electrode.

-. talllliif'" This invention relates to electron beam furnaces and, more particularly, to an electron beam heating system in such a furnace for heating a target therein.

Electron beam furnaces have been used for some time in the vacuum processing of various materials. Such furnaces .are utilized, for example, in the melting and casting of metallic ores to obtain relatively pure metals .or alloys. Such furnaces are also used in the melting of materials other than metals, such asceramics and plastics, and are frequently used to produce vapors of metals and othermat erials for deposition upon a substrate.

Electron beam furnaces utilize one or more electron beam heating systems in which electron beam guns are used for producing high energy electron beams. These beams are then directed in some manner to a target for heating the same, generally by means of magnetic fields. Electron beam guns generally comprise a heated electron source or emitter for emitting the electrons, and suitable means for generating a magnetic field for accelerating and focusing the electrons into a beam. The interior of the furnace is usually evacuated to a very low pressure and the electron beam gun is disposed at v a convenient location within the vacuum chamber.

A particularly advantageous type of electron beam gun utilies an elongated filament or emitter .(of tungsten or other suitable thermionically emissive material) disposed in an elongated channel in a backing or focusing electrode. The filament is heated toa thermionically emissive temperature and the backing electrode is maintained ata suitable negative potential in order to direct the electrons produced by the filament out of the open side of the channel. Suitable means may then be provided for accelerating and directing the electons to a target. In co-pending U. S. application Ser. No. 103,684, now US. Pat. No. 3,172,007 assigned to the assignee of the present invention, an electron beam gun utilizing two elongated filaments in a single channel is described. This particular construction is of advantage in avoiding collision of positively charged ions with the emitters, which could produce erosion and shorten filament life.

In electron beam furnaces which are utilized for evaporating material for deposition on a substrate, one of the most prevalent problems is that of spitting from the skulled crucible in which the melt is contained. Spitting is a frequently major reason for rejection of coated parts when the droplets impinge upon and adhere to the substrate being coated. -Where electron beam power is increased to increase the evaporation rate, spitting frequently becomes an even greater problem.

It is an object of the present invention to provide an improved electron beam heating system for use in an electron beam furnace.

Another object of the invention is to provide an electron beam heatingsystem for an electron beam furnace used to heat a material contained within a skulled crucible in which spitting is substantially reduced.

It is another object of the invention to provide an electron beam gun for an electron beam heating system which provides a beam of high power and low density.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with'the accompanying drawings wherein:

FIG. 1 is a schematic view illustrating'an electron beam furnace in which an electron beam heating system is used forheating a material contained within a skulled crucible;

FIG. 2 is a top view of an electron beam gun used in a heating system constructed in accordance with'the invention; and g FIG. 3 is a side view, partially in section along the line 33.ofFIG. 2,0f the electronbeam gun of'FIG. 2.

Very generally, the electronbeam heating system of A the invention-comprises a focusingelectrode l1 defining an elongated open-sided channel "12. Means 13 are provided for connecting the focusing electrode to a first sourceof potential. Atleastone accelerating electrode 14 is spaced from the open side of the channel defined by the focusingelectrode. Means 15 are :provided for connecting the accelerating electrode to a source'of potential which is substantially more positive than the first source. At least four and less than nine elongated filaments 16 of thermionically emissive material areprovided Means 17'and 18 supportthefilaments parallelwith each other'within the channel extendingalong'the length thereof and spaced from the focusing electrode. Means 19connect the filaments to a source of heating current to produce thermionic emission of electronsfor accelerating bythe accelerating electrodes.

Referring now more particularlyto FIG. 1, a portion of an electron beam "fumace is illustrated schematically. The electron beam furnace includes an enclosure, 'notshown, and means, also not'shownffor'supporting a substrate to be coated at a suitable location above a crucible 21. The crucible 21 is provided with a plurality of coolant passages 23 therein through which a fluid coolant, such'as' water, is circulatedjA melt 25, contained'within the crucible 2l,is heatedby means of an electron beam 27 to produce a vaporfor deposition on the substrate, now shown. Because of'the cooling of the crucible 2l,a skull 29 of solidified moltenmaterial existsbetween the molten pool 25 and the crucible 21.

In order to heat' the'material in themolten pool 25, an electron beam gun 31 is provided. Theelectron beam gunis'of a construction subsequently described and produces the electronbeain27. The electron beam 27 is steered by magnetic fields through an arcuate pathof about 270 toimpinge upon the surface of'the molten pool 25. In order to produce the magnetic fields, two or more pole pieces 33 are provided, suitably polarized by electromagnet or permanent magnet means, now shown. As-the electrons pass through the transverse magnetic fieldcreated "between the pole pieces 33, the elctrons are deflected through the desired arcuate path.

Referring to FIGS. 2 and 3, the details of the construction of the electron beam gun 31 may be more clearly seen. The gunincludes a'base plate 35 which is suitably mounted in the interior of the electron beam furnace system. Mounted to the base plate 35, approximately in the center thereof, is 'the'block-shapedfocusing electrode 11. The focusing electrode is of any suitable electrically conductive material upon which ahig'h negative potential is maintained. The block 11 may be electrically insulated from "the *base :plate 35, or the base plate 35 may be electrically insulated from the furnace structural elements to which it is mounted, thereby isolating the block 11 from ground potential. The electrical connection is made to the block 11 by any suitable means, such as a connector bracket 13, for maintaining the block at the desired negative potential.

The side of the block 11 opposite the base plate 35 is provided with an elongated open-sided channel 12. In the illustrated embodiment, the channel 12 runs the entire length of the block 11 and is open-ended at each end. As will be described subsequently, the filaments 16 are mounted in the channel 12 and are heated to a temperature suitable for the emission of electrons. The electrons emitted by the filaments 16, due to the negative potential of the block 11, flow out of the open side of the channel.

In order to accelerate the electrons leaving the open side of the channel 12, an accelerating electrode is provided comprising a pair of accelerator rods 14. The rods 14 extend between accelerator mounting bars 37 parallel with each other and on opposite sides of the channel 12. Suitable electrical connection means, such as a bracket 15, are provided to maintain the rods 14 at a potential which is substantially more positive than the potential of the block 11 and the filaments 16. In this manner, the electrons are accelerated into a beam, passing through the space between the rods 14 to be deflected by the magnetic field and impinge upon the melt 25 (FIG. 1).

The accelerator bars 37 are mounted across the partially open ends of a top plate 41. The top plate 41 is supported on three high voltage insulators 43, mounted on mounting posts 45 to the base plate 35. Shield cups 47 are also mounted on the insulators 43, protecting them from flash-over. Suitable nuts 49 hold down the top plate 41 against the insulators 43. Similar nuts 51, one of which is visible, clamp the mounting post 45 to the base plate 35.

A cathode shield 53, of generally L-shaped cross section, extends across the top of the channel 12 above the filaments. A lock pin 55 for the cathode shield extends outwardly from the cathode shield between two of the insulators 43. The cathode shield is maintained at a potential close to that of the block 11, thereby blocking electrons in the central part of the beam. This makes the beam more diffuse for more uniform heating, but is not essential to the invention.

Returning now to FIG. 1, it may be noted that the area of the surface of the melt 25 upon which the electron beam 27 impinges, extends over substantially the entire surface of the melt, but is a substantial distance from the skull 29. In achieving uniform evaporation and, in the case of alloy, in minimizing segregation problems, it is desirable that a uniform heat pattern exists on the surface of the melt 25. Moreover, it is also desirable that the electron beam not impinge close to or upon the interface between the skull 29 and the pool 25. If the latter occurs, spitting is much more pronounced and a deleterious effect on the coated product may result.

In order to achieve a uniform heat distribution, several techniques are available. For example, a large area indirectly heated cathode may be utilized. Such a system, however, requires a separate electron beam heating system for the cathode itself, decreasing efficiency and requiring a much greater power level of operation. Typically, the indirectly heated cathode type of gun is used with axial fields, rather than transverse fields as shown in FIG. 1. In the case of axial fields, the erosion problem due to ions of positive charge being accelerated along fields to strike the emitter is a disadvantage.

Linear filament systems offer certain advantages due to their greater simplicity, ease of manufacture and replacement, and generally greater power per watt of cathode heat than indirectly heated cathode. Linear filament systems, however, suffer from a problem in that because the filament produces a narrow ribbon-shaped beam, defocusing of the beam to achieve a wide uniform impact area may be difficult.

In accordance with the present invention, the linear filament principle is utilized, but a relatively high num ber of linear filaments are placed in the same cathode block recess. The effect is that of a plurality of closely adjacent ribbon-shaped beams which diffuse into one another due to the space charge effect to produce a very uniform impact area on the surface of the crucible. A very high power level is achievable by utilizing the number of filaments in accordance with the invention, while at the same time, concentrations of beam power in small areas, particularly near the skull-melt interface, are easily avoided.

Returning now to FIGS. 2 and 3, the particular illustrated embodiment employs six linear filaments 16. The filaments are mounted parallel with each other extending the length of the channel 12. Three filaments are on each side of the longitudinal axis plane of the channel. In this way, erosion problems are minimized, since any positive ions being accelerated in the channel will be focused into the space between the middle two filaments at the longitudinal axis plane of the channel.

In order to support the filaments, three filament straps 56 are utilized. The filament straps are mounted to clamping blocks 57 by means of screws 59. A plurality of insulators 61 are provided for maintaining insulation between the straps 56 and the block 11, and a bracket 19 is provided for supplying electrical current to the straps 56. The upper ends of each strap 56 is provided with an elongated slot 63. The slot 63 in each strap divides the strap into two portions, each of which is provided with a slot at the top through which a filament 16 passes. The filaments 16 are provided with stops 65 at each end thereof which abut the outer side of the filament straps 56 at the slots 66 therein. The filament straps are biased outwardly in order to provide tension on the filaments 16 by applying a bias force to the stops 65.

The electrical current path for the filaments may be of any suitable configuration but, in the illustrated embodiment, the filaments are connected in groups of two, with each of the groups being in series. The filament strap adjacent the bracket 13 is not insulated from the block 11, thereby providing a filament current path from the bracket 19 to the bracket 13 through the filament pairs.

When heating current is supplied to the filaments, electrons are emitted and, due to the wide area from which they are emitted, a beam of a substantial cross section emerges from the recess or channel 12 and is accelerated by the accelerator rods 14. A beam of substantial power and of substantial cross section and with a high degree of uniformity is thus produced. This beam is easily controlled to impinge upon the surface of the melt 25 in the crucible over substantially the entire surface, a substantial distance from the skull 29. In

this way, spitting is minimized, but maximum heating power and uniformity are also achieved.

Although six filaments are shown, the invention includes the use of four to eight filaments. Less than four filaments is not particularly useful in achieving high power levels with maximum diffuseness. Greater than eight filaments provides diminishing returns since the addition of a filament does not provide advantages coincident with increased fabrication and operating costs. Moreover, if the accelerator rods have to be spaced apart too widely, greater potentials must be used to accelerate.

It may therefore be seen that the invention provides an improved electron beam heating system for heating a material contained within a skulled crucible in an electron beam furnace. The electron beam heating system of the invention minimizes the likelihood of spitting while enabling a relatively high level of power to be placed into the melt, thus achieving a correspondingly high evaporation rate,

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are, intended'to fall .within theiscope of the appended claims. a What is claimed is: i

,;1. In anelec tron beam furnace, an electron'beam heating system for heating a target therein, comprising,

6 a focusing electrodedefining an elongatedopen-sided channel, means for connecting said focusing electrode to a first source of potential, at least one accelerating electrode spaced from the open side of said channel defined by said focusing electrode, means for connecting said accelerating electrode to a source of potential which is substantially more positive than said first source, at least fourand less than nine elongated linear filaments of thermionically emissive material, means for supporting said filaments parallel with, each other within said channel extending along the length thereof and spaced from said focusing electrode, and means for connecting said filaments to a source of heating current to produce thermionic emission of electrons for acceleration by said accelerating electrode.

2. An electron beam heating system according to claim 1 wherein the target comprises material in a skulled crucible, including magnetic guidance means for directing and focusing the accelerated electrons to impinge uniformly over most of the surface of the material in the crucible a substantial distance from the skull.

3. An electron beam heating system according to claim 1 including an even number of filaments, half of which are supported on each side of the longitudinal axis plane of said channel.

4. An electron beam heating system according to claim I" wherein the number of said filaments-is six. 

1. In an electron beam furnace, an electron beam heating system for heating a target therein, comprising, a focusing electrode defining an elongated open-sided channel, means for connecting said focusing electrode to a first source of potential, at least one accelerating electrode spaced from the open side of said channel defined by said focusing electrode, means for connecting said accelerating electrode to a source of potential which is substantially more positive than said first source, at least four and less than nine elongated linear filaments of thermionically emissive material, means for supporting said filaments parallel with each other within said channel extending along the length thereof and spaced from said focusing electrode, and means for connecting said filaments to a source of heating current to produce thermionic emission of electrons for acceleration by said accelerating electrode.
 2. An electron beam heating system according to claim 1 wherein the target comprises material in a skulled crucible, including magnetic guidance means for directing and focusing the accelerated electrons to impinge uniformly over most of the surface of the material in the crucible a substantial distance from the skull.
 3. An electron beam heating system according to claim 1 including an even number of filaments, half of which are supported on each side of the longitudinal axis plane of said channel.
 4. An electron beam heating system according to claim 1 wherein the number of said filaments is six. 